عن عبد الله بن عمر- رضي الله عنهما- مرفوعاً:«أن النبي صلى الله عليه وسلم نهى عن بيع الوَلاءِ وعن هِبَتِهِ». [صحيح] - [متفق عليه] المزيــد ...
‘Abdullāh ibn ‘Umar (may Allah be pleased with him) reported: The Prophet (may Allah's peace and blessings be upon him) prohibited the selling and gifting of Walā' (the right of a master to inherit from his freed slave by virtue of the bond of loyalty between them). Sahih/Authentic. - [Al-Bukhari and Muslim]
Walā', or the bond of loyalty between a master and his freed slave, is similar to the bond of blood relationship between kin, in the sense that it cannot be obtained through selling or gifting. That is why it is not permissible to act freely concerning it via sale or otherwise.
It is a kind of tie or bond between the emancipating party and the emancipated slave, by virtue of which the former is entitled to inherit from the latter. Therefore it is prohibited to sell this bond, because it is similar to blood relationship that cannot be removed by someone’s action.
That is, if someone were to try to sell his blood relationship with his brother, son or paternal cousin, the sale would be invalid. Likewise, selling the loyalty bond between a master and his freed slave is invalid.